The Role of Beta-Endorphin in Horses: a Review

Veterinarni Medicina, 56, 2011 (9): 423-429 Review Article

The role of beta-endorphin in horses: a review

M. Golynski1, W. Krumrych2, K. Lutnicki1

1Faculty of Veterinary Medicine, University of Life Sciences, Lublin, Poland 2National Veterinary Research Institute, Pulawy, Poland

ABSTRACT: Opium alkaloids counterparts are secreted by human and animal organisms but the role of endogenous opioid peptides in horses has not yet been fully elucidated. Endogenous opioids are involved in regulating food intake, sexual and social activity, pain relief and pain threshold regulation in horses as well as in regulating the functions of the immune system. The aim of this review is to describe the endogenous opioid system in the horse and its function during stress, illness, reproduction, and its influence on immunity and on the formation of reactive oxygen species (ROS) in horses. What is currently known concerning beta-endorphin suggests that they can be a promising diagnostic or prognostic indicator of many pathologic states in horses.

Keywords: horse; endorphin; opioids

Contents

1. Introduction 6. The role of beta-endorphin in immunity 2. Secretion 7. Prooxidative/antioxidative balance 3. Beta-endorphin and stress 8. Conclusions 4. Beta-endorphin in illness 9. References 5. Beta-endorphin and the reproductive system

1. Introduction endorphins and dynorphins. These bind to the fol- lowing specific opioid receptors types: MOR (m), Alkaloids originating from opium, used through- KOR (k) and DOR (d), and to the untypical orphan out centuries as common anaesthetics, are still receptor NOR causing numerous biological effects widely used nowadays. It has long been presumed in the entire organism as well as in specific organs. that these substances may have counterparts, which Endogenous opioids are involved, among others, are secreted by animal and human organisms. This in regulating food intake as well as sexual and social notion was supported by the research of Kosterlitz activity. They also exhibit pain relieving properties and Hughes, who in 1975 described the presence and regulate pain threshold in horses, similarly to of the first endogenous ligands of opioid receptors; the way they do in people (Hamra et al., 1993). The however, the role of endogenous opioid peptides in pain relieving function of opioid receptors evolved animals has not yet been fully explained (Kosterlitz as an element of the organismal defence system and Hughes, 1975). against stress. This function consisted primarily in There are presently many substances included in regulating the immune system (Stefano et al., 1998). the group of endogenous opioids, of which the most Thus, opioids may be ascribed the function of im- widely researched and best known are encephalins, munomodulators.

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2. Secretion the appearance of the stressor. The authors suggest that beta-endorphin modulates the activity of the Proopiomelanocortin (POMC) is a precursor of hypothalamic-pituitary-adrenal axis, thereby regu- one of the endogenous opioid peptides – beta-en- lating ACTH secretion. Yet, the evidence brought dorphin, which is secreted in the horse by the pars forward in support of this claim is inconclusive. It intermedia of the pituitary gland. Additionally, proo- may cautiously be stated that beta-endorphin release piomelanocortin is a substance from which two oth- from the pituitary gland in horses is synchronised ers are formed: adrenocorticotropin (ACTH 18-39, with the initial phase of the stress reaction and may corticotropin-like intermediate lobe peptide – CLIP) in this way mitigate the negative results of cortisol and α-melanotropin (α-melanocyte stimulating on the organism. hormone, α-MSH) (Scott and Miller, 2003). Beta- endorphin is the strongest endogenous opioid along with the m- (MOR) and d-opioid (DOR) receptor 4. Beta-endorphin in illness ligand. The daily rhythm of beta-endorphin secretion One of the best examples of hyperbetaendorphine- is similar to that of ACTH and cortisol. The high- mia in horses is ECS (Equine Cushing’s Disease). est values of opioid were noted in morning blood Lowe (1993) reports that changes in the behaviour of samples (peak level at 9.00) (Hamra et al., 1993). affected individuals such as lethargy or submissive- Different results were obtained by Mehl et al. (1999), ness may result from increased concentrations of be- who did not note statistically significant differences ta-endorphin. According to Millington et al. (1988), in the levels of beta-endorphin examined in mares the concentration is 60 times higher in the plasma every two hours between 8 a.m. and 8 p.m. Owing to and 120 times higher in the cerebrospinal fluid of the inconsistency of research results so far, the data ECS-affected animals. Another cause of changes in published by Pell and McGreevy (1999) concerning animal behaviour connected with the opioid system the activity of beta-endorphin in horses with ster- is described by authors who compared the levels of eotypy, and the results of the study on foals by Fazio endorphins in healthy horses and horses with ster- et al. (2009) should be interpreted with caution, tak- eotypy (Pell and McGreevy, 1999). They argue that ing into consideration such factors as seasonality. At the diseased horses exhibited a congenital increased present, data on the daily rhythm of beta-endorphin sensitivity of opioid receptors. However, the claim is secretion in horses remains very limited. not fully proved as it is only based on the observation that there is a lack of statistically significant differences in beta-endorphin concentrations between 3. Beta-endorphin and stress healthy and diseased horses. The concentration of beta-endorphin in blood The release of beta-endorphin into the blood in was also assayed in the case of colic diseases. Recent horses is particularly evident during the course of research conducted by a group of Finnish scholars stress reactions. A study on 42 healthy pure blood focussed on, among other questions, specifying the stallions was aimed at determining the impact of levels of circulating beta-endorphin using the RIA transport stress on the levels of beta-endorphin, method in colicky horses. Additionally, the study cortisol and ACTH (Fazio et al., 2008). An increase was concerned with assessing the usefulness of the in the levels of circulating ACTH was observed after method as an indicator of pain intensity, prognosis travelling distances of 100 and 200 kilometres and and stress (Niinisto et al., 2009). Observations were levels of cortisol were higher after traversing dis- made of 77 diseased horses divided into three groups tances of 100, 200 and 300 kilometres. However, the according to the intensity of clinical symptoms – mild, concentration of beta-endorphin was raised when moderate and acute. Groups were not uniform and compared to the basic level only after the distance included many horses with only intestinal obstruc- of 100 kilometres. After the next 100 and 200 kilo- tion and displacement. The control group consisted metres a decrease was observed which may suggest of 15 clinically healthy horses. Only ponies were ex- an effect of negative feedback signalling. It may cluded from the study; therefore, it may be assumed thus be assumed, on analysing the levels of beta- that horses of different types were used. Among the endorphin and ACTH that the release of the opioid experimental groups a significant variability of results into the blood occurs maximally one hour after was observed due to the duration of the disease and 424 Veterinarni Medicina, 56, 2011 (9): 423-429 Review Article the different ages of the animals in question. It ap- although reports concerning horses are scarce. The pears justified to trace the level of beta-endorphin in impact of beta-endorphin on the activity of the re- patients belonging to fairly uniform age groups and productive system remains largely a point of theo- relate the results to such parameters as the duration retical debate, based mainly on comparisons drawn of the disease. Moreover, many animals included in between different species. The role of the peptide in the study had been treated with pain relievers (mainly stallions, which is secreted by Leydig’s cells in the flunixine meglumine) prior to hospitalisation. The testicles/testes, may be limited to suppressing the influence of the treatment on the results of the study activity of Sertoli cells (Roser, 2008). This stands was not explained by the authors. Therefore, it seems in partial contrast to the findings of other authors, sound to examine only the horses which did not un- who state that although POMC gene expression in dergo any pain alleviation treatment. The concentra- epididymides and testes was not observed in stal- tions of beta-endorphin in cured animals were lower lions, the presence of beta-endorphin was found in than in the deceased ones as well as animals which these organs using immunocytochemical staining had to be euthanized due to the lack of effect of the (Soverchia et al., 2006). These results may suggest treatment (12.29 and 34.57 pmol/l, respectively). The plasma as the source of the peptide rather than assessment of the peptide concentration may have testes or epididymides, and are not sufficient, in a vast prognostic significance. In healthy horses the the light of the current state of knowledge, to fully value was 5.71 pmol/l, while in the diseased animals explain the role of beta-endorphin in the function- with mild, moderate and acute clinical symptoms ing of the reproductive system in stallions. the values were 11.43, 14.0 and 32.29 pmol/l, respec- The role of endogenous opioids in females ap- tively. The results of the examination conducted af- pears to be complex. Reliable research, employing ter the animals were brought to hospital, with the naloxone – an opioid receptor blocker, was con- abovementioned influence of transportation on the ducted in mares (Behrens et al., 1993). The authors beta-endorphin concentration in plasma taken into assert that opioid peptides influence FSH and LH account, may not be fully reliable. The measurement release in the luteal phase of the ovarian cycle and of beta-endorphin levels is therefore only useful in the may play a role in suppressing gonadotropin se- case of non-transported horses, that is, if samples are cretion by progesterone. It is interesting that an collected prior to the appearance of transportation increase in FSH and LH levels in the luteal phase of stress, on the site of colic occurrence. A high correla- the cycle after applying naloxone was observed both tion of the obtained results with the intensification of in breastfeeding and non-breastfeeding mares. clinical symptoms, including heart rate and mortal- A pool of circulating beta-endorphin was stud- ity as well as ACTH and cortisol levels, constitutes ied in women during the menstrual cycle. No evidence supporting the usefulness of determining significant fluctuations in beta-endorphin levels the level of beta-endorphin in horses with colic symp- were noted during the cycle. In addition to this, toms as a prognostic indicator. The levels of opioid in no relationship was found between its concentra- plasma rises in the case of colic with endotoxemia, tion in blood and estradiol, progesterone, lutein- probably by means of serotoninergic mediation, as izing hormone and foliculotropin levels, despite the a shock effect (Niinisto et al., 2009). The obtained fact that ovaries contain considerable amounts of results may confirm the notion that measuring the beta-endorphin. This leads to the supposition that concentration of plasma beta-endorphin is beneficial ovaries exert no influence on the pool of circulat- in monitoring visceral pain (McCarthy et al., 1993) ing beta-endorphin (Martyn et al., 1986). Similar and orthopaedic pain in horses (Raekallio et al., 1997), results, but with regard to beta-endorphin in the as beta-endorphin is an endogenous pain relieving uterus and the placenta, were obtained in the case substance secreted in response to stress, similarly to of sows. A raised level of beta-endorphin in plasma ACTH (Niinisto et al., 2009). was observed during pregnancy, but not during the menstrual cycle, and its release during labour may be induced by prostaglandin F2-alpha (Aurich et 5. Beta-endorphin and the reproductive al., 1993). system In women, increased secretion of beta-endorphin during pregnancy may stimulate the foetus to adapt Beta-endorphin has also been a subject of in- to extrauterine life, and some amount of it may quiry for researchers in the field of reproduction, be transferred from the mother’s bloodstream to 425 Review Article Veterinarni Medicina, 56, 2011 (9): 423–429 the foetus (Radunovic et al., 1992). In a study con- the inflammation site and in circulating leukocytes ducted in rats, it was determined that pregnancy (Mousa et al., 2003). Furthermore, beta-endorphin and labour are connected with a considerable rise may play a major role in the course of artificially in beta-endorphin levels in the hypothalamus, the induced peritonitis in mice in which it was detected midbrain and the amygdala when compared to in exudate (Chudzinska et al., 2005). It has a con- non-pregnant animals (Wardlaw and Frantz, 1983). siderable significance in natural pain control, which The results suggest that beta-endorphin of central may be completely neutralised through immuno- origin influences the level of pain perception and suppression (Janson and Stein, 2003). It appears the behaviour of a female during pregnancy. Beta- then that there is a great deal of evidence imlicating endorphin, as a modulator of behavioural events, is beta-endorphin as an immunomodulator and in thought to have a role in shaping perinatal activity some situations it may oppose the effects of cor- in females (including mares). tisol. Therefore, the abovementioned changes in beta-endorphin levels during stress reactions may be extremely important in contributing to balance 6. The role of beta-endorphin in immunity of the immune system in horses.

The results of research conducted so far indi- cates that there is a correlation between endog- 7. Prooxidative/antioxidative balance enous opioids and the immune system. However, data and observations pertaining to this correlation In recent years there have been numerous papers in horses are very limited. published concerning free-radical processes and The impact of endogenous opioids on receptors their role in the emergence and the course of many distributed in the central nervous system may lead diseases, both in humans and in animals (Droge, to anti-inflammatory effects as a result of a suppres- 2002; Marlin et al., 2002). The scale of danger sion of cytokine secretion, as spleen macrophages posed by excess reactive oxygen species (ROS) is in mice whose ability to produce beta-endorphin manifested in the fact that they participate in the has been disabled secrete significantly more IL-6 etiopathogenesis of around one hundred disorders, after LPS stimulation than the cells of animals who including cancers, cardiovascular and degenerative are able to produce beta-endorphin (Refojo et al., diseases (Bartosz, 2005). It is suspected that free 2002). Additionally, human peripheral blood poly- radical damage which accumulates throughout life morphonuclear cells (PMN), regardless of TNF-α is the cause of ageing processes (Ji et al., 1998). stimulation, bred in the presence of beta-endor- Therefore, the influence of endogenous opioids on phin, exhibit greater apoptosis than cells incubated the formation of reactive oxygen in horses deserves in the absence of opioids. Furthermore, the effect further attention. is reversible by naltrexone – an opioid antagonist Owing to their intensive oxygen metabolism and (antagonist of opioid receptors) (Sulowska et al., their use in sport, horses appear to be especially 2002). sensitive to the consequences of induced oxidative The immunomodulating effect of opioids is com- stress, as during physical effort the demand for oxy- plicated by the fact that numerous inflammatory gen increases by up to 60 fold. It is widely known cells exhibit the ability to produce endogenous that in cell respiration at most 98% of the oxygen opioid peptides, which is of great significance for supplied is used. The remaining 2% undergoes in- the process of inflammatory reactions. In rats with complete reduction, leading to the formation of free experimentally induced inflammation an infiltra- radicals in the entire organism. Broadly speaking, tion of leukocytes containing beta-endorphin was the free radicals in question are reactive oxygen observed (Machelska et al., 2003). These cells re- species (ROS), which are extremely active chemi- lease the peptide in the inflammatory focus, which cally and react with cell constituents. The ROS in- may not only have an immunomodulating, but also clude, among others, hydrogen peroxide (H2O2), pain-relieving effect. This is clearly visible after the singlet oxygen ([1]O2), hypochlorous acid (HOCl) •– administration of corticoliberin (CRH) to rats with and superoxide anion radical (O2 ), as well as hy- • •– artificially induced inflammation. This supports droxyl radical ( OH), hydroperoxil radical (HO2 ) the co-expression of beta-endorphin and cortico- and nitrous oxide (NO•). Reactive oxygen species, liberin receptors in inflammatory cells infiltrating principally hydroxyl radical and singlet oxygen, 426 Veterinarni Medicina, 56, 2011 (9): 423-429 Review Article cause damage to cellular structures through oxida- one, effectively suppresses this activation (Sharp et tion and reduction reactions and through the pro- al., 1985). In research conducted in rats, a significant duction of organic radicals. Reactive oxygen species influence of beta-endorphin has been observed on mainly attack plasma membrane lipids, where the the expression of both induced and endothelial nitric peroxidation of polyunsaturated and esterified fatty oxide synthase (iNOS and eNOS respectively) dur- acids takes place, which leads to disturbances in ing ovulation. NOS activity was excited indirectly network structure and the loss of transport abilities through inhibiting the production of prostaglandins, (Dinis et al., 1993). Another site of impact for ROS and the effect was reversible after administering nal- are proteins which may directly undergo denatura- trexone (Faletti et al., 2003). This suggests that the tion, fragmentation, cross-linking and aggregation. abovementioned problems may also affect horses, in Furthermore, reactions may take place indirectly, which the formation of reactive oxygen species could in the presence of peroxides (Davies, 1993). With be of critical importance, especially in the conditions regard to carbohydrates, a disruption of glycosidic of considerable physical burden. bonds between monomers may occur as well as an increase in their proneness to hydrolysis. Damage processes may affect both carbohydrate structures 8. Conclusion of glycolipids and glycoproteins, which may lead to antigen changes on the cell surface (Hunt et The opioid system is a subject of attention not al., 1988; Bartosz, 2004). Free radicals (hydroxyl only due to its complexity but also its impact on radicals in particular) also react with nucleic acids, key functions of the organism. There is little known damaging their structure, disrupting DNA threads concerning beta-endorphin as a promising diag- or even entire chromosomes (Breen and Murphy, nostic/prognostic indicator in horses is and not all 1995). Importantly, reactive oxygen species destroy research results published so far allow the draw- cytotoxic and suppressor T lymphocytes, which ing of definite conclusions. For these reasons we may result in autoimmune phenomena (Walport propose that the questions raised above require and Duff, 1998). As far as blood cells are concerned, further scientific attention. respiratory burst, characterised by increased production of reactive oxygen species, applies mainly to neutrophils, and the evaluation of their oxygen 9. 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Corresponding Author: Marcin Golynski, University of Life Sciences, Faculty of Veterinary Medicine, Department and Clinic of Animal Internal Diseases, Sub-Department of Internal Diseases of Farm Animals and Horses, Lublin, Poland E-mail: [email protected]

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